A valid model of visual function requires the correlation of perceptual phenomena observed psychophysically with appropriate physiological substrates. To this end we intend to study single units in cat striate cortex under the same experimental conditions used to produce psychophysical after-effects which by inference are likely to be mediated by the striate cortex. The after-effects to be studied reflect adaptation to motion, orientation and size, all spatial properties of the adapting stimulus. Not only will these experiments demonstrate the role of cortical single units in producing these after-effects, they will also address the question of the processes underlying the remarkable spatial selectivity of cortical receptive fields. In preliminary experiments we have found that some spatial properties of cortical neurons can be adapted differentially, enabling the functional dissection of the receptive field into subunits selective for stimulus size or direction of motion. We also intend to explore the actual physiological mechanisms responsible for spatial adaptation of cortical cells by recoding intracellularly during the adaptation process. By this means it should be possible to resolve whether the adaptation is pre- or post-synaptic and whether it involves a reduction of excitation, increased inhibition or both. Clarification of these issues will produce a more complete understanding of how neurons in striate cortex integrate information, and will help to reveal general patterns of organization within the brain.